Electrical powered devices are subjected to numerous conditions which can result in fire, injury by burning, injury by electrical shock, or the release of toxic fumes. For example, aging or mechanical damaged electrical insulation can produce local short-circuits, and overheating and perhaps ignition of the surroundings because of these electrical short-circuits. Electrical overloads and overheating and potentially ignition of surrounding combustibles can be caused by electrical motors which have either seized, or been mechanically blocked from rotating, or deprived of the necessary cooling flow. Electrical wiring, cable and power cords can be damaged by rough treatment, carelessness, or exposure to a harsh or corrosive environment.
Numerous safety devices have been disclosed previously. Present practice is to have a circuit breaker which will open the circuit if a preselected current is exceeded for a selected period of time and, in some high risk circuits, to utilize a ground fault protection circuit breaker which will open the electrical circuit, if any of the circuit""s current leaks to ground. Other electrical safety devices have been proposed but have not received widespread use perhaps because of excess cost, complexity, electrical, or mechanical limitations as well as concerns about the safety of the device itself. Many fires are caused by improper or loose electrical connections, which create high electrical resistance and an over-temperature condition which results in electrical fires, but do not exceed the electrical current limits of the circuit breaker or involve a leakage of electrical current to ground.
Other safety devices sense the temperature at selected points along the electrical cord or at points within the electrical component or components. For example, fusible links which melt and open an electrical circuit upon over-temperature conditions have been proposed and have received limited use. Other devices employ thermistors, RTDs, or other temperature sensitive elements, which, in conjunction with a sensing and control circuit, monitor the temperatures of the sensor and reduce or cutoff the power to the effected device if the sensor overheats. Because these devices detect overheating only at certain points, these safety devices protect only at discreet locations. Dangerous overheating conditions at unprotected points may go undetected. In addition, the protection of a long electrical power cord is not feasible, using this discreet technology, because of the very large number of sensors required to protect such a large distributed surface area.
Modern complex machinery adds additional requirements for electrical power distribution, control wiring, and communication. For example, a modern aircraft utilizes numerous circuits which require a high level of reliability and safety. Routine inspections alone are not sufficient to ensure that electrical wiring and cables are in satisfactory condition. Much of the wiring is inaccessible, and visual inspection may not indicate deterioration of insulation.
Mechanical damage or thermal or chemical deterioration may lead to current paths from energized conductors through the insulation in the wiring or cable. Ignition of the insulation may form additional low resistance paths to ground or other return paths, leading to a condition known as arc tracking. Since damage to the wiring and nearby components may be extensive, a method is needed to indicate deterioration of the wire before it is significant enough to cause an arc.
Therefore an object of the present invention is to provide a fault sensing wire and alarm apparatus capable of real time sensing the condition of a large number of wires, cables, and power cords, and providing indication of unsafe conditions before severe faults occur.
Another object of the present invention is to provide a fault sensing wire and apparatus which detects an overtemperature condition along any portion of the wire.
Another object of the invention is to provide a fault sensing wire and apparatus which detects mechanical damage to the wire which might result in a short.
Another object of the invention is to provide multiple fault sensing wires and apparatus which allows monitoring of the multiple fault sensing wires for unsafe conditions.
Another object of the invention is to provide multiple fault sensing wires and apparatus which discriminates between the severity and type of potential faults in the sensor wire.
Another object of the invention is to provide a fault sensing wire and apparatus which utilizes one or more environmental sensors to modify setpoints of a controller.
Another object of the invention is to provide a fault sensing wire and apparatus which is capable of detecting low level arcing conditions.
The electrical safety device of the present invention comprises a fault sensing wire and apparatus which monitors the condition of the wire and provides indication, alarms or appropriate actions to prevent or mitigate unsafe electrical conditions. The fault sensing wire comprises an electrical conductor and a sensor strip distributed in the insulation surrounding the conductor. The sensor strip comprises a thermal responsive element and a mechanical damage responsive element. The temperature responsive element comprises a material distributed along the substantial length of the fault sensing wire which has a positive coefficient of resistivity which increases with temperature. The mechanical damage responsive element is a conductive material distributed along the substantial length of the fault sensing wire disposed between the conductor and as much of the outside surface of the wire as practical. In this manner, an object which cuts, abrades, or frays the outside surface of the wire will cut or open the mechanical damage responsive element before the external object contacts the conductor. The temperature responsive element and the mechanical damage responsive element functions may be performed by a single sensor strip, or the functions may be performed by a plurality of sensor strips.
In one embodiment, the sensor strip is a conductive polymer having a positive temperature coefficient of resistivity which increases with temperature. The temperature-resistance response of such a material results in a response region in which the resistance increases slowly with temperature until a xe2x80x9cswitchxe2x80x9d region is reached. After this xe2x80x9cswitchxe2x80x9d region, the resistance increases rapidly with temperature. This xe2x80x9cswitchxe2x80x9d action results in the ability to detect relatively short fault regions in the wire.
Another embodiment distributes the sensor strip in a helical pattern in the insulation around the conductor. In this way, the sensor strip performs the functions of the temperature responsive element and the mechanical damage sensing element, which are in fact, the same element. In other embodiments, a plurality of thermal responsive elements are disposed longitudinally in the insulation and spaced radially about the conductor. By connecting the ends of the thermally responsive elements together with shunts, the series connected matrix of thermally responsive elements forms the sensor strip and performs both thermal sensing and mechanical damage sensing functions. In still other embodiments, the mechanical damage responsive element may be a metallic wire disposed in a helical pattern around the conductor, and the thermal responsive element may be a conductive polymer strip. The thermal responsive element and the mechanical damage sensing element may be connected in series to form the sensor strip.
To provide control functions, the sensor strip of a fault sensing wire is connected to a control unit. The control unit comprises an impedance or resistance measuring circuit and an output unit. The control unit may be a comparator which compares the impedance of the sensor strip to a set or reference value. If the measured impedance exceeds the set value, the output of the comparator changes states and activates an alarm, a circuit trip element such as a breaker, or a relay performing some other action based on the measured value of the sensor strip impedance.
Due to the non-linear response of the conductive polymer having a positive temperature coefficient of resistivity which increases with temperature, the magnitude of the impedance of the sensor strip provides an indication of the thermal and/or mechanical condition of the wire. For example, a resistance in the range less than the xe2x80x9cswitchxe2x80x9d area of the resistance-temperature curve may represent an overtemperature, but less than damaging condition in the wire. An appropriate output may be an alarm activating signal. The resistance in the range above the xe2x80x9cswitchxe2x80x9d area may indicate a potential damaging overtemperature. An appropriate output may be a trip of a circuit breaker providing current to the conductor. For an open in the sensor strip (xe2x80x9cinfinitexe2x80x9d resistance), indicating mechanical damage such as fraying of the wire, an appropriate output may be a signal to an alarm indicating that inspection of the wiring is necessary. Still another resistance reading of near zero may indicate a sort or other improper condition of the apparatus and initiate a different alarm. Several comparators with different setpoints may be used to monitor the same fault sensing wire to activate different outputs depending on the magnitude of the impedance. Logic circuits or a microprocessor may be used to select the appropriate response based on the magnitude of the measured impedance.
Another embodiment utilizes a multiplexer to sample sensor strips in a plurality of fault sensing wires. A microprocessor may be used to sample each sensor strip and compare a measured impedance to one or more setpoints stored in the microprocessor. An environmental sensor such as an external temperature sensor may used to adjust the setpoints in the memory of the microprocessor to enhance the response of the apparatus under varying conditions.